To prevent the non-authorised piratical utilization of the digital contents, some solutions provide for the transmission to the user of digital audiovisual contents which is protected, scrambled and generally ciphered by one or several encoding key(s). The descrambling thus depends on the reception of a certificate or a licence which contains one or several descrambling key(s), the certificate or licence being generally sent to the user in exchange for a charged or free transaction. The certificate or licence generally contains legal elements relating to the possible utilization of the descrambled contents. One problem entailed in such solutions is that a user having powerful tools at their disposal can determine the descrambling key or keys without going through the transaction, and thus obtain a descrambled stream which can be used as desired, either for an illegal visualization or audio, or for making pirated copies. Another problem is that the user must have a terminal which incorporates the descrambling method as well as the so-called rights of use management method compatible with that used for scrambling the contents.
Some protective solutions enable the secure distribution of audiovisual contents through telecommunication networks or on audiovisual physical supports of the memory card or DVD types, more particularly using protection methods called DRM (Digital Rights Management) or CAS (Conditional Access System). Further in the following text, all the DRM and CAS will be called “DRM.”
A DRM is a protection method implemented with a protection device comprising:                protection means of a cryptographic kind;        digital rights management means;        means for the secure transmission of keys of the cryptographic kind.        
However, several DRMs exist, among which some are standardized. However, the DRMs are incompatible with each other. A consumer receiving audiovisual contents protected by one DRM can play it only if the terminal has the method corresponding to the DRM: the interoperability of the contents and the interoperability of the protection systems (DRM, CAS) do not exist.
EP 0 778 513 discloses a method making it possible to prevent the illegal use of information by adding a control information to check the rights of the user. The system makes it possible to know permanently what part of the information is used, by which user, and thereby makes it possible to know whether the user is in a legal or illegal situation. Such method thus protects data by adding additional information which change the nature of the initial information, but the interoperability with the other DRM systems is not provided.
WO 00/49483 also provides methods and systems for creating a link between the users and a digital entity editor. The method comprises at least one of the following steps: the step of subdividing the digital entity into two parts; the step of storing a part in a memory area of a server connected to a computer network; the step of transmitting the other part to at least one user having a computer; the step of connecting the computer to the computer network; the step of establishing a functional link between the first part and the second part. Such methods and systems do not mention whether the part stored in the server can be stored by the user which would enable him to pirate the digital entity, and the interoperability with the other DRM systems is not mentioned.
U.S. Pat. No. 5,937,164 discloses separating a stream into two parts, the smaller one of which contains information required for using the bigger one. However, this method is not sufficient for answering the problem identified. As a matter of fact, the elimination of a part of the stream changes the format of the stream which can no longer be recognized as a standard stream, which could be used with general software applications. That method requires both a specific software on the server side for separating the two parts, and other specific software not only providing the reconstruction of the stream but also the acquisition of the main stream and the use thereof according to a solution proprietary format. Such proprietary format is not the initial format of the stream before it was separated into two parts. Interoperability with the other DRM systems is not provided.
The OMA (Open Mobile Alliance) consortium and more particularly, in the specifications of the OMA 2.0 DRM does not make it possible to solve the problem identified, since OMA defines how the original audiovisual sequence is completely ciphered, then how the rights are structured in a security object prior to being sent to the decoders. Interoperability with the other DRM is not solved by OMA.
Two possibilities exist to avoid such drawbacks: either the terminals have all the existing DRMs, and the terminals can thus remove the protection on and play contents protected by all the present DRMs, but the terminals will not be able to play the contents protected by unknown DRMs which will be used in the future; or the audiovisual contents to be protected are protected by all the existing and future DRMs, the contents having to be protected and broadcast with all the DRMs, which means extremely important protection and broadcasting costs, without allowing the interoperability of the DRM systems, nor the free exchange of files between terminals, since each terminal does not have all the DRMs.
In addition, some terminals such as personnel digital assistants or mobile telephones cannot have several DRMs for reasons of software and hardware architecture.
Thus, in the existing solutions, if it is desired to transmit audiovisual contents comprising several types of DRM protections, it is necessary to transmit the whole corresponding stream which corresponds to the audiovisual contents, protected according to the various DRM formats. The volume of information thus transmitted is thus equal to the size of the contents to be visualized multiplied by the number of the DRM types used. However, the transmission of such volume of information is a problem, more particularly in case of a transmission, because of the limited passbands and memories.